1. Field of the Invention
This invention relates to gasphase ion sources for time-of-flight mass-spectrometers with any number of electrodes for the acceleration of ions and with electrodes capable of generating transverse electrical fields for changing the transverse velocity component of charged particles.
In a time-of-flight mass-spectrometer a point in time is defined, called start-time, when a group of ions is started on their path. At the end of a drift space the time is measured which an arriving ion has needed on its flight and this time is used to determine the mass of that ion.
The extraction volume is that region within the ion source of the mass-spectrometer, from which, upon start-time, ion paths lead to the surface of the detector of the time-of-flight mass-spectrometer. The paths of the ions are given by the electrical fields and the physical laws of motion within.
The start-time of time-of-flight analysis can be given by:
the point of time, when neutral particles of a gas are ionized within the extraction volume by a laser or electron beam crossing it. PA1 the point of time when the electrode voltages of the ion source are switched on. This is usually the case when ions are to be analysed, since ions can only reach the extraction volume, when the voltages on the electrodes of the ion source are switched off. PA1 An ion source that focuses transverse velocities: This type of ion source is used when the distribution of velocities in the analyte gas or ion beam is large. This type of ion source tries--independent of initial transverse velocities--to bend all ion paths as parallel to the ion optical axis as possible. This type of ion source is not the subject of this invention and will not further be discussed here. PA1 An ion source with deflection field: This ion source is often used, when the distribution of initial velocities within the analyte gas or ion beam is small. Since all ions need their transverse velocities changed by a very similar value, a transverse field is necessary, whose strength is independent of transverse coordinates. This type of ion source is the subject of this invention as given by the generic terms of claim 1. PA1 The chapter "III. Results, A. Time-of-flight mass spectrometer" in the publication of Dietz et al. (Journal of Chemical Physics, volume 73(10), pages 4816-4821, 1980) expains a mechanism that suppresses an unwanted signal that can be caused by residual gas particles. Residual gas particles will always be present in the ion source for vacuum technical reasons. PA1 The mass range of the ion source can be limited from above and below by applying static voltages to the deflection electrodes. FIG. 2 in the publication of Rohlfing et al. (Journal of Physical Chemistry, volume 88, pages 4497-4502, 1984) shows how it is possible to select different mass regions by changing the voltages on the deflection electrodes. PA1 Applying a time-dependent voltage to the deflection electrodes, it is possible to transport a significantly larger mass range into the time-of-flight mass-spectrometer. This mass range is only limited by apertures along the paths. This option is described in a publication of Lubman and Jordan (Review of Scientific Instruments, volume 56(3), pages 373-376, 1985). PA1 Ions, whose initial velocity in the direction of acceleration is zero, should have a final velocity in the direction of acceleration that depends exclusively on the initial coordinate in the direction of acceleration. In particular, the final velocity in the direction of acceleration should be independent from initial coordinates in transverse directions and initial velocities in transverse directions. Such a behaviour can be induced by a homogeneous accelerating field. PA1 After passing a homogeneous acceleration field the velocity components in transverse directions will not have changed. The transverse velocity components are independent of the starting point of the ions, which means that they are also independent from the coordinate location after passing the accelerating field. As a consequence, to change the transverse velocity components, an electric field is necessary, whose field strength in transverse directions is independent of the value of the transverse coordinate values.
The ion optical axis of a gasphase ion source is understood as the path of one selected ion. The path of this ion starts with the initial velocity v=0 at start-time of mass analysis from some conveniently chosen point close to or at the geometric midpoint of the extraction volume. If the construction of the ion source is rotationally symmetric, the starting point of the ion optical axis is usually chosen on the axis of symmetry.
To achieve a high mass resolution in a time-of-flight mass-spectrometer with gasphase ion source, the initial velocity components in the direction of acceleration within the ion source must be kept small. This can be done by injecting the analyte gas or ion beam at right angles to the direction of acceleration into the ion source. The publication of Bergmann et al. (Review of Scientific Instruments, volume 60(4), pages 792-793, 1989) expains why this right angle is necessary and how in this manner a mass resolution of 35000 (m/.DELTA.m) FWHM (Full Width at Half Maximum) can be achieved. There are two types of ion sources that have the direction of the analyte gas or ion beam not parallel to the direction of acceleration within the ion source:
A transverse electric field is understood here as an electric field whose field vector points in transverse direction. The strength of this transverse field should only have a minor dependance on the coordinate values in transverse directions. This electric field is termed deflection field, the electrodes that produce such a field are termed deflection electrodes.
2. Description of the Related Art
Aside from the possibility of achieving higher mass resolutions, gasphase ion sources corresponding to the generic terms of claim 1 have a number of further advantages:
The physical facts leading to state-of-the-art ion source constructions are as follows:
All implementations known so far have separatly arranged acceleration and deflection fields, i.e. the deflection field is always arranged after the acceleration field. Usually the transverse electric field is generated by a parallel plate capacitor. In all these ion sources the mass range is limited from above, because heavy ions drift too far away from the ion optical axis before reaching the deflection field and thus are lost on apertures etc.
Taking all the above advantages of having the direction of the analyte gas or ion beam and the direction of acceleration in the ion source at right angles, the mass range limitation just named is a serious drawback.